Ovarian cancer is the fourth leading cause of cancer deaths among women in the United States and causes more deaths than all other gynecologic malignancies combined. In the United States, a woman's lifetime risk of developing ovarian cancer is 1 in 70. In 1992, about 21,000 cases of ovarian cancer were reported, and about 13,000 women died from the disease. [Chapter 321, Ovarian Cancer, Harrison's Principles of Internal Medicine, 13th ed., Isselbacher et al., eds., McGraw-Hill, New York (1994), pages 1853-1858; American Cancer Society Statistics, Cancer J. Clinicians, 45:30 (1995). Epithelial ovarian cancer, the most common ovarian cancer, has a distinctive pattern of spread: in addition to metastasis through the lymphatic and blood vessels to areas such as the liver, lung and brain, cancer cells may also migrate through the peritoneum to produce multiple metastatic nodules in the visceral and parietal peritoneum and the hemidiaphragms. Early stage ovarian cancer is often asymptomatic and is detected coincidentally by palpating an ovarian mass on pelvic examination. In premenopausal patients, about 95% of these masses are benign. Even after menopause, 70% of masses are benign but detection of any enlargement requires exploratory surgery. In postmenopausal women with a pelvic mass, a markedly elevated serum CA-125 level of greater than 95 U/ml indicates malignancy with a 96% positive predictive value. [Chapter 321, Ovarian Cancer, Harrison's Principles of Internal Medicine, supra.]
Epithelial ovarian cancer is seldom encountered in women less than 35 years of age. Its incidence increases sharply with advancing age and peaks at ages 75 to 80, with the median age being 60 years. The single most important risk factor for this cancer is a strong family history of breast or ovarian cancer. In families in which ovarian, breast, endometrial or colon cancer can be tracked as an apparent autosomal dominant trait, the risk of this cancer can be as high as 50%. Having a single first-degree relative with ovarian cancer increases a woman's risk by at least three-fold, and having a personal history of breast or colorectal cancer increases the risk of subsequently developing ovarian cancer by two-fold. [Chapter 321, Ovarian Cancer, Harrison's Principles of Internal Medicine, supra.] In addition, those with identifiable genetic mutations in genes such as BRCA1 also have an increased risk. Baker et al., Etiology, Biology, and Epidemiology of Ovarian Cancer, Seminars in Surgical Oncology 10: 242-248, 1994; Amus et al., Genetic Epidemiology of Epithelial Ovarian Cancer, Cancer 71: 566-72, 1993; Whitmore, Characteristics Relating To Ovarian Cancer Risk: Implications for Preventing and Detection, Gynecologie Oncology 55, 515-19, 1994. Oncogenes associated with ovarian cancers include the HER-2/neu(c-erbB-2) gene, which is overexpressed in a third of ovarian cancers, the fms oncogene, and abnormalities in the p53 gene, which are seen in about half of ovarian cancers. A number of environmental factors have also been associated with a higher risk of epithelial ovarian cancer, including a high fat diet and intake of lactose in subjects with relatively low tissue levels of galactose-1-phosphate uridyl transferase.
In epidemiological studies, behavior associated with decreased ovulation, such as pregnancy, breastfeeding and use of estrogen-progestin combination oral contraceptive medications, decrease the risk of ovarian cancer; use of estrogen-progestin combination oral contraceptives for as long as 5 years can reduce the risk of ovarian cancer by 50%. Greene et al., The Epidemiology of Ovarian Cancer, Seminars Oncology, 11: 209-225, 1984; Whitmore et al., Characteristics Relating To Ovarian Cancer Risk: Collaborative Analysis of 12 US Case-Control Studies, American J. Epidemiology 136: 1212-20, 1992. Conversely, early menarche, late menopause and nulliparity (no pregnancies) have been shown to increase the risk of ovarian cancer. The risk has been shown to positively correlate with the number of ovulatory cycles in a woman's lifetime. Wu et al., Personal and Environmental Characteristics Related To Epithelial Ovarian Cancer, American J. Epidemiology, Vol. 108(6) 1216-1227. The long-term use of ovulation-inducing ovarian hyperstimulants such as clomiphene has been shown to be associated with an increased risk of ovarian cancer in some women. Rossary et al., Ovarian Tumors in a Cohort Of Infertile Women, New Engl. J. Med., 331: 771-6, 1994. Thus, some factors that favor prolonged and persistent ovulation have been thought to increase ovarian cancer risk, whereas some factors that suppress ovulation have been thought to decrease risk. [Chapter 321, Ovarian Cancer, Harrison's Principles of Internal Medicine, supra.] These data have led to the “incessant ovulation” hypothesis for the development of ovarian cancer. Casagrande et al., “Incessant Ovulation” and Ovarian Cancer, Lancet at 170-73 (Jul. 28, 1979). This hypothesis is that repeated ovulation cycles, each of which involves the disruption and repair of the ovarian surface epithelium, may cause neoplastic transformation of the ovarian epithelium in susceptible individuals and that the risk of ovarian cancer is associated with the number of ovulation cycles in a woman's lifetime.
There is no established pharmaceutical approach to the prevention of ovarian cancer. For all women, especially those at high risk of developing this disease, the only option available at this time is surgical removal of the ovaries, with all of the attendant risks and subsequent adverse health consequences due to resulting estrogen deficiency.
Although epidemiological evidence suggests that the use of combination oral contraceptives, which contain both an estrogen and a progestin, is associated with a subsequent reduced risk of developing epithelial ovarian cancer, the mechanism for this protective effect is unknown, and oral contraceptive preparations are not currently approved for this purpose. The reduction in risk of ovarian cancer in women who have used estrogen-progestin combination oral contraceptives for at least three years is approximately 40 percent. Moreover, this protective effect increases with the duration of use and persists for up to two decades after discontinuation of use. Rosenberg et al., A Case Control Study of Oral Contraceptive Use and Invasive Epithelial Ovarian Cancer, The WHO Collaborative Study of Neoplasia and Steroid Contraceptives; Epithelial Ovarian Cancer and Combined Oral Contraceptives, Int'l J. Epidemiology 18: 538-45, 1989; Lee et al., The Reduction in Risk of Ovarian Cancer Associated with Oral Contraceptive Use, New Engl. J. Med. 316: 650-51, 1987; Thomas P. Gross, James J. Schlesselman, The Estimated Effect of Oral Contraceptive Use on the Cumulative Risk of Epithelial Ovarian Cancer, Obstetrics Gynecology 83: 419-24, 1994; Franceschi et al., Pooled Analysis of 3 European Case-Control Studies of Epithelial Ovarian Cancer: III. Oral Contraceptive Use, Int'l J. Cancer 49: 61-65, 1991.
It is commonly believed that the protective effect of oral contraceptives is related to the ability of these drugs to inhibit ovulation. Estrogen-progestin combination oral contraceptives act primarily by suppressing the pituitary gland's production of gonadotropins, thereby inhibiting the hormonal stimulus for ovulation. These combination drugs also have direct inhibitory effects on the reproductive tract, including inducing changes in the cervical mucus that decrease the ability of sperm to enter the uterus, as well as changes in the endometrium that reduce the likelihood of implantation, and reducing fallopian tube motility and uterine secretions.
The epidemiological studies showing the protective effect of combination oral contraceptives evaluated older combination preparations which typically contained higher doses of drug than most contraceptive regimens used today. Common older regimens contained 50 micrograms or more of ethinyl estradiol (an estrogen) or 100 micrograms or more of mestranol (an estrogen) and greater than 1 mg of norethindrone, norethindrone acetate or norethynodrel (a progestin). Table 1 infra lists the progestin and estrogen content of some older regimens. All of the currently used low-dose combination oral contraceptives contain lower doses of both progestin and estrogen, as well as a lower ratio of progestin to estrogen. Consequently, it has not been definitively established that the newer low-dose combination oral contraceptives are associated with the same protective effect as the older high-dose combination contraceptives. Rosenblatt et al., High Dose and Low Dose Combined Oral Contraceptives: Protective Against Epithelial Ovarian Cancer and The Length of the Protective Effect, Eur. J. Cancer, 28: 1870-76, 1992.
Despite the overall safety of combination oral contraceptives, their use is not recommended for women smokers older than age 35, for women of all ages who are at increased risk for myocardial infarction, for women with liver disease, and for women older than age 40. Serious and potentially fatal side effects include deep vein thrombosis, pulmonary emboli, myocardial infarction, thromboembolic stroke, hemorrhagic stroke, and high blood pressure. In the 35-39 year old age group, the use of oral contraceptives among women smokers doubles their risk of death. After age 40, the mortality rate even in non-smoker women using oral contraceptives (32.0 per 100,000) is greater than women using no contraception (28.2 per 100,000), while the mortality rate for smoker women is quadrupled (117.6 vs. 28.2 per 100,000). [Chapter 340, Disorders of the Ovary and Female Reproductive Tract, Harrison's Principles of Internal Medicine, supra, pages 2017-2036.]
Progestin-only contraceptives do not reliably inhibit ovulation, but are nevertheless contraceptively effective, presumably due to direct effects on the reproductive tract. The actual contraceptive mechanism of action is unclear. Prior epidemiological studies have exhibited no consistent pattern of either increasing or decreasing risk of ovarian cancer according to duration of use. The WHO Collaborative Study of Neoplasia and Steroid Contraceptives Depot-Medroxyprogesterone Acetate(DMPA) and Risk of Epithelial Ovarian Cancer, Int'l J. Cancer. 49:191-195 (1991); Liam et al., Risk of Breast, Uterine, Corpus, and Ovarian Cancer in Women Receiving/Medroxyprogesterone Injections, J. Am. Med. Ass'n 249:2909-2912 (1983). Thus, unlike the data available for progestin-estrogen combination contraceptives, the prior art relating to progestin-only contraceptives does not suggest that the use of a progestin reduces the risk of epithelial ovarian cancer.
Estrogen, alone or with low doses of progestin, is also used as hormonal replacement therapy in menopausal women. For long term use, Premarin® (conjugated equine estrogen) is generally given at a dose of 0.625 mg orally daily (equivalent to 10 to 20 μg ethinyl estradiol orally per day) or an equivalent dose transdermally. Other regimens add cyclic progestins or continuous low-dose progestins, typically 2.5 to 10 mg per day of Provera® (medroxyprogesterone acetate). One epidemiologic study has suggested that hormone replacement therapy with estrogen alone may be associated with an increased risk of developing ovarian cancer. Rodriguez et al., Estrogen Replacement Therapy and Fatal Ovarian Cancer, Am. J. Epidemiology, 141:828-835 (1995).